This invention relates generally to optically pumped lasers, and, in particular to a mercury laser which incorporates therein an RF excited mercury discharge lamp for optically pumping the lasing medium.
The optically pumped mercury laser has been in existence for a relatively short period of time. This laser has many potential applications as, for example, a general purpose low power visible laser, a laser wavelength standard or for laser gyroscope applications.
A typical optically pumped mercury laser utilizes a water jacketed laser tube having conventional Brewster windows mounted on greaseless O-ring taper joints. High reflectivity mirrors are used to form a long-radius optical cavity. The laser tube and the thin deionized water jacket are generally made of fused silica. Extremely pure tank nitrogen is flowed into the laser tube over a drop of natural isotopic-ratio Hg placed in the bottom of a small U-tube bend. The N.sub.2 -Hg mixture is pumped through the laser tube at slow flow rates by a trapped mechanical pump. The Hg vapor pressure is controlled by controlling the temperature of the U tube, with the laser tube itself being kept slightly warmer to avoid Hg condensation. Optical pumping of the laser tube is accomplished by two standard Hg germicidal lamps in a polished double-elliptical pump cavity.
An operative example of such an optically pumped mercury laser is set forth in an article by Max Artusy, Neil Holmes and Anthony Siegman entitled "DC Excited and Sealed-Off Operation of the Optically Pumped 546.1-nm Hg Laser", Applied Physics Letters 28, 133-134 (Feb. 1, 1976).
Various types of mercury discharge lamps have been extensively developed for more than 50 years. Mercury lamps are widely available in commercial models, both as low-pressure glow discharge lamps which are efficient producers of ultraviolet light, used for germicidal lamps and other purposes; and as high-pressure arc-discharge lamps used as efficient visible light sources. The properties of mercury discharge lamps of different types are extremely diverse.
For pumping the mercury laser one requires a mercury discharge lamp that emits only two of the many characteristic mercury discharge lines, at 2537 A and 4047 A, with high efficiency and brightness but with a very narrow bandwidth for each of those lines. In general, the physical requirements for achieving all of these purposes at once are contradictory. Increasing the discharge current and/or the gas pressure in conventional mercury lamps increases the radiative efficiency and the total light output from the discharge. However, the spectral width (bandwidth, linewidth) of the emitted lines generally increases at the same time, and a phenomenon known as "self-reversal" generally appears. This phenomenon refers to the property that the discharge lamps at higher pressures and currents actually emits less radiation and becomes less bright at the exact center frequency of the radiative transitions, while emitting much more energy in the wings of the transitions, that is to say, at wavelengths located slightly to either side of the exact transition frequencies.
The optically pumped mercury laser, however, is effectively pumped only by radiation which is exactly at the center wavelengths of the two transition frequencies mentioned. Self-reversal must therefore be avoided or eliminated in lamps for pumping this laser.
Cataphoresis is another significant problem in mercury discharge lamps. As the discharge current is increased, particularly at low pressures where self-reversal might be avoided, the mercury vapor is transported or pumped by the discharge current itself in a direction from the anode end to the cathode end of the discharge, where the mercury condenses and freezes out. At higher discharge currents this phenomena can rapidly transfer all of the mercury in a lamp to the cathode end, and thus terminate the operation of the lamp.
Furthermore, conventional dc current excited mercury lamps suffer both electrode deterioration due to sputtering and other phenomena, and discharge instabilities and noise fluctuations due to processes that are inherent to dc discharges.
It is therefore clearly obvious from the problems set forth hereinabove that the development of an optically pumped mercury laser utilizing an optical pumping source which alleviates these problems is becoming a necessity.